The-present invention refers to a method for atomic absorption spectroscopy of an analyte which is contained in a sample to be analyzed and which is converted into free atoms in an absorption volume of an atomizer, said method comprising the steps of position- and time-dependent measuring of the atomic absorption over the cross-section of the absorption volume, and simultaneous determination of surface temperatures of the atomizer.
Furthermore, the present invention refers to a device for carrying out an atomic absorption spectroscopy, comprising a radiation source, an atomizer enclosing an absorption volume, a position- and time-resolving spectrometer for measuring the light which has been emitted by the radiation source and which has passed through the absorption volume, and a means for determining the surface temperature of the atomizer.
Such a method and such a device are known from WO 94/23285.
This known method especially includes the step of first converting an analyte of a sample to be examined in the absorption volume of a graphite furnace into the gaseous phase in the form of free atoms. These free atoms absorb characteristic wavelengths emitted by a primary light source. The extinction of the primary radiation source in these wave-lengths caused by said free atoms is then measured in a temporally resolved manner, i.e. at specific moments of time, in dependence upon the position relative to the cross-section of the furnace.
According to this method, the position- and time-dependent extinctions are first determined for various calibration standards whose concentrations are already known. Accordingly, a connection, i.e. a calibration function, between the extinction and the concentration of the analyte observed is obtained in the sample to be measured.
For determining the concentration of an unknown sample, the position- and time-dependent extinction is, subsequently, determined, and the time-dependent concentration corresponding to the measured extinction is ascertained through the calibration function.
In this connection, non-specific radiation losses (background absorption) can be corrected by background correction methods which are normally used in the field of atomic absorption spectroscopy, such as correction with a continuum radiator, Zeemann effect background correction or correction according to the Smith-Hieftje method. In accordance with the requirements of the correction technique used, data evaluation is temporally controlled in such a way that a distinction can be made between the respective signals (e.g. non-specific absorption, total absorption, emission) coming from the atomizer. In any case, however, also the background absorption is measured in a temporally resolved manner so that correction errors caused by inhomogeneous background absorption, whose distribution can, moreover, be different from that of the atomic absorption, can be avoided, such correction errors being unavoidable in the case of the conventional atomic absorption which is not spatially resolved.
Furthermore, the surface temperature of the atomizer can be measured simultaneously in this method. Additional information on the atomization process can be obtained in this way.
According to WO 94/23285, spatially local deviations, which would result in a non-linearity of the calibration function in their entirety, are taken into account by the use of a spatially-resolving type of atomic absorption spectroscopy. Hence, many calibration measurements must be carried out in this non-linear region so as to obtain a satisfactory, reliable determination of the connection between the measured extinction of the atoms within the absorption volume and the concentration of the analyte in the sample to be measured. These spatially local deviations include, for example, the inhomogeneous distribution of the atoms in the absorption volume, the inhomogeneous distribution of the radiation intensity in the absorption volume and the temperature gradients in the absorption volume.
The method and the device according to WO 94/23285 are, however, disadvantageous insofar as spectral effects, such as Doppler broadening and pressure broadening effects, which depend on the temperature, are not taken into account.
Hence, it is the object of the present invention to improve the known method and the known device.
According to the present invention, this object is achieved by a method of the type mentioned at the beginning which is characterized by the steps of reconstructing the temperature field in said absorption volume on the basis of the surface temperatures determined, determining position- and time-dependent numbers of particles of the absorbing atoms of the analyte on the basis of the measurements of the position- and time-dependent atomic absorption and the absorption profile that has been determined with due regard to effects influencing the line profile of the analyte and with due regard to the reconstructed temperature field, and determining the time-dependent total number of the absorbing atoms of the analyte on the basis of the position- and time-dependent numbers of particles.
It follows that the method according to the present invention first reconstructs, for each predetermined time step, the temperature field over the cross-section of the absorption volume on the basis of the surface temperatures of the atomizer which are determined at the support points or support values. Accordingly, the temperature is known at any point over the cross-section of the absorption volume. With the aid of this position-dependent temperature, position-dependent absorption profiles are determined in which spectral effects, such as Doppler broadening and pressure broadening effects, have already been taken into account. These position-dependent absorption profiles can then be used for determining with the aid of absorption measurements position-dependent numbers of particles on the basis of which the total number of the absorbing atoms of the analyte can then be determined for each predetermined time step.
Due to the fact that spectral effects, such as Doppler broadening and pressure broadening effects, are directly taken into account upon determining the position-dependent numbers of particles, said position-dependent numbers of particles and, consequently, also the total number of absorbing atoms can be determined with a degree of accuracy which is much higher than that obtained by methods according to the prior art. This direct taking into account has especially the effect that systematic deviations, which are caused by the above-mentioned effects, are reduced significantly.
A further advantage of the method according to the present invention results from the fact that the experimental set-up used for carrying out the measurement only has to be taken into account upon reconstructing the temperature field in the absorption volume on the basis of the surfaces temperatures determined. When the manner in which the temperature field is to be reconstructed on the basis of the surface temperatures has been determined for a specific experimental set-up, any type of analytes can be examined by means of this set-up without any necessity of measuring calibration standards. This results in a certain degree of independence of the measurements from the set-up used in the device in question.
It follows that, in addition to an improvement of the measuring accuracy, the method according to the present invention also makes the execution of these measurements much simpler.
Measurements of the position- and time-dependent absorption are corrected in an advantageous manner with regard to the above-mentioned background absorption.
According to a special embodiment of the present method, the the position- and time-dependent particle number N(X,t) can be determined from             ∫              Δ        ⁢                  xe2x80x83                ⁢        λ              ⁢                  ⅆ        λ            ⁢              xe2x80x83            ⁢              J        ⁡                  (                      λ            ,            X                    )                    ⁢              ⅇ                              -                          k              ⁡                              (                                  λ                  ,                  T                                )                                              ⁢          f          ⁢                      xe2x80x83                    ⁢                      N            ⁡                          (                              X                ,                t                            )                                            =                    10                  -                      A            ⁡                          (                              X                ,                t                            )                                          ⁢                        ∫                      Δ            ⁢                          xe2x80x83                        ⁢            λ                          ⁢                              ⅆ            λ                    ⁢                      xe2x80x83                    ⁢                      J            ⁡                          (                              λ                ,                X                            )                                            =          Φ      ⁡              (                  X          ,          t                )            
where xcex is the wavelength (integration variable), xcex94xcex is a spectral bandpass of the spectrometer used for measuring the atomic absorption, J(xcex,X) is an a priori known emission profile of the primary radiation source used for measuring the atomic absorption, k(xcex,T) is an a priori known temperature-dependent absorption profile, f is the oscillator strength of a transition observed, A(X,t) is the position- and time-dependent extinction, and "PHgr"(X,t) is the position- and time-dependent intensity of the radiation of the primary radiation source which passed through the absorption volume (i.e. which was not absorbed in the absorption volume), said intensity being determined by measurement.
Due to this representation of the absorption profile, it is possible to use various physical models for the absorption profile k(,T). In this respect, empirically determined models, i.e. models which have essentially been determined on the basis of measurements, can be used on the one hand and models which are known from theory on the other. The theoretically known models can, if necessary, also be optimized with regard to the experimental conditions in question. In particular, also dependencies on further physical parameters, which influence the absorption, can be taken into account in a comparatively simple manner in this representation.
Furthermore, this method also takes into consideration a spatially inhomogeneous intensity distribution of the respective primary radiation source used. It follows that a requirement which had to be fulfilled in the field of atomic absorption spectroscopy up to now, viz. the necessity of providing a homogeneous radiation intensity over the cross-section of the absorption volume, need no longer be satisfied. Hence, it is, on the one hand, no longer necessary to satisfy this demand by providing an appropriate experimental set-up. On the other hand, the accuracy of measurements will, of course, be increased according to the present invention, since errors, which were introduced in the measurement according to the prior art due to deviations from the above demand, no longer occur.
The emission profile J(xcex, X) and the temperature-dependent absorption profile k(xcex, T) can, in an advantageous manner, be determined a priori from the following formulae:             J      ⁡              (                  λ          ,          X                )              =                            ∑                      k            =            1                    n                ⁢                  xe2x80x83                ⁢                              b            k                    ⁢                                    H              k                        ⁡                          (                                                                    λ                    -                                          Δ                      ⁢                                              xe2x80x83                                            ⁢                                              λ                        k                                                                              α                                ;                                  a                  e                                            )                                          +                        J          s                ⁡                  (                      λ            ,            X                    )                                k      ⁡              (                  λ          ,          T                )              =                  ∑                  k          =          1                n            ⁢              xe2x80x83            ⁢                        b          k                ⁢                                            H              k                        ⁡                          (                                                λ                  -                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                                          λ                      k                                                        +                                      Δλ                    s                                                  ;                a                            )                                .                    
Where: k is the k-th hyperfine structure component of the transition observed, xcex94xcexk is the position of the k-th hyperfine structure component, Js(xcex, X) is the profile of the spectral scattered light component of the primary radiation source, xcex94xcexs is the pressure broadening of the absorption profile relative to the emission profile, Hk (xcex; . . . ) is the Voigt profile of the k-th hyperfine structure component in which the Doppler broadening and the pressure broadening are taken into account, a and ae are the ratios of the Doppler broadening and the pressure broadening component for the absorption profile and the emission profile, xcex1 is the factor by which the emission profile is narrower than the absorption profile, and bk are the relative standardized intensities of the individual hyperfine transitions.
In accordance with this preferred embodiment, a quasi-classical model is assumed for the absorption profile and the emission profile; in said model, the transitions taking place in an atom, including the hyperfine structure transition, can be taken into account individually. This model is taken into account by a sum of the Voigt profiles generally known in the field of spectroscopy, said Voigt profiles including a Gaussian Doppler broadening and a Lorentz pressure broadening component. It follows that it is especially also possible to take into account the influence of each individual hyperfine structure transition on the measurement results.
The representation of the emission profile of the primary radiation source chosen hereinbefore has the advantage that the influence of scattered light of the primary radiation source on the measurement result can be taken into account directly. Also this circumstance will improve the measurement results in comparison with the known method.
The method according to the present invention can be used in connection with a great variety of different atomizers known in the field of atomic absorption.
For example, a known graphite furnace with or without integrated platform can be used. In this case, the temperatures of the atomizer surface can be determined by pyrometric measurements at support points defined in the area of the atomizer wall and, where applicable, in the area of the integrated platform. For this purpose, the radiation intensity of the temperature radiation emitted by the components in question is measured and, finally, converted into a temperature. Details describing how such a temperature measurement can be carried out are disclosed e.g. in WO 94/23285.
In addition, for specific analytes to be detected, e.g. mercury, the method can also be used in connection with an experimental set-up including a quartz cell, which can be operated in a heated (e.g. for the hydride technique) as well as in an unheated (e.g. for the cold-vapour technique) condition. In the latter case, the temperature measurements are reduced to a measurement of the room temperature.
Further advantageous embodiments of the method according to the present invention result from the description of the preferred embodiments following hereinbelow as well as from the dependent method claims.
The method according to the present invention can be carried out by a device of the type mentioned at the beginning, which is characterized by a means for reconstructing the temperature field in the absorption volume on the basis of the surface temperatures determined, a means for determining absorption profiles with due regard to effects influencing the line profile of the analyte and with due regard to the reconstructed temperature field, a means for determining the position- and time-dependent numbers of particles of the absorbing atoms of the analyte on the basis of the position- and time-dependent measurements of the spectrometer and the absorption profiles determined, and a means for determining the time-dependent total number of particles of the absorbing atoms of the analyte on the basis of the position- and time-dependent numbers of particles.
This device according to the present invention can be further developed in an advantageous manner in accordance with the specific embodiments of the methods used. These advantageous further developments also result from the description of the preferred embodiments as well as from the dependent device claims.